A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to impart a beam of radiation with a pattern in its cross-section corresponding to a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In optical lithography, an alternating Phase Shift Mask (altPSM) may be used to print, for example, a pattern of lines and spaces. Compared to the use of a conventional Chrome on Glass mask (COG mask) for printing a pattern of lines and spaces, an improved process latitude and a reduced sensitivity to mask CD error may be obtained. The lines of a line-space pattern on an altPSM are typically lines of an absorbing material such as chromium. Neighboring transmissive regions on each side of a line have a same transmittance, but have a different mask thickness. One of the two transmissive regions is modified to have a mask thickness different from (e.g. thinner than) the mask thickness of the other (unmodified) region. The mask thickness difference is chosen such that it corresponds to half a wavelength of illumination radiation as used for the imaging. As a result, two sections of an illumination beam have, upon traversing two respective, neighboring transmissive regions of the mask pattern, a phase difference of 180°. Phase-shifted radiation having traveled through the modified transmissive regions provides an interference with radiation emanating from unmodified transmissive regions of the mask. The interference has the effect of improving a contrast of the image of the pattern of lines and spaces on the substrate. Such a contrast improvement may ultimately increase a lithographic process window.